1. Field of the invention
The present invention relates to an electroluminescent (EL) device having an emitting layer made of an emitting substance, which utilizes an electroluminescence phenomenon that the emitting substance emits light by applying an electric current to the emitting layer. More particularly, it is concerned with an organic EL device in which the emitting layer is made of an organic emitting substance.
2. Description of the prior art
As prior art organic EL devices, there have been known a device of a two-layer structure having two layers of organic compounds as shown in FIG. 1, in which an organic fluorescent thin film 3 (hereinafter referred as "emitting layer") and an organic hole transport layer 4 are laminated with each other and are arranged between a metal cathode 1 and a transparent anode 2. There have been also known a device of three-layer structure having three layers of organic compounds as shown in FIG. 2, in which an organic electron transport layer 5, an emitting layer and an organic hole transport layer 4 are laminated in sequence and are sandwiched as a whole between a metal cathode 1 and a transparent anode 2. The hole transport layer 4 facilitates the infusion of the holes from the anode and blocks electrons. The electron transport layer 5 facilitates the infusion of electrons from the cathode.
In these organic EL devices, a glass substrate 6 is furnished outside the transparent anode 2. The recombination of electrons infused from the metal cathode 1 and the holes infused from the transparent anode 2 to the emitting layer 3 generates excitons. The excitons emit light when they are deactivated through radiation. This light radiates toward the outside through the transparent anode 2 and the glass substrate 6.
The conventional organic EL devices constructed as indicated above generally emit light even at a low voltage. However, when the EL device with a simple matrix structure is continuously driven by a DC current, its luminance is likely to deteriorate and then the EL device easily becomes destructible. The heat resistance of such a device is also low.